A commercially available electrolyte capacitor as a rule is made of a porous metal electrode, an oxide layer serving as a dielectric on the metal surface, an electrically conductive material, usually a solid, which is introduced into the porous structure, an outer electrode (contacting), such as e.g. a silver layer, and further electrical contacts and an encapsulation. An electrolyte capacitor which is frequently used is the tantalum electrolyte capacitor, the anode electrode of which is made of the valve metal tantalum, on which a uniform dielectric layer of tantalum pentoxide has been generated by anodic oxidation (also called “forming”). A liquid or solid electrolyte forms the cathode of the capacitor. Aluminium capacitors in which the anode electrode is made of the valve metal aluminium, on which a uniform, electrically insulating aluminium oxide layer is generated as the dielectric by anodic oxidation, are furthermore frequently employed. Here also, a liquid electrolyte or a solid electrolyte forms the cathode of the capacitor. The aluminium capacitors are usually constructed as wound- or stack-type capacitors.
π-conjugated polymers are particularly suitable as solid electrolytes in the capacitors described above because of their high electrical conductivity. π-conjugated polymers are also called conductive polymers or synthetic metals. They are increasingly gaining economic importance, since polymers have advantages over metals with respect to processability, weight and targeted adjustment of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes), a particularly important polythiophene used industrially being poly(3,4-ethylenedioxythiophene) (PEDOT), since it has a very high conductivity in its oxidized form.
The solid electrolytes based on conductive polymers can be applied to the oxide layer in various ways. EP-A-0 340 512 thus describes, for example, the production of a solid electrolyte from 3,4-ethylenedioxythiophene and the use thereof in electrolyte capacitors. According to the teaching of this publication, 3,4-ethylenedioxythiophene is polymerized on to the oxide layer in situ. In addition to the in situ polymerization described above, processes for the production of solid electrolytes in capacitors in which a dispersion comprising the already polymerized thiophene, for example the PEDOT/PSS dispersions known from the prior art, is applied to the oxide layer and the dispersing agent is then removed by evaporation are also known from the prior art.
Important properties of a capacitor are, inter alia, its low temperature properties and its life. “Low temperature properties” of a capacitor are understood as meaning the influencing of the electrical characteristic values thereof, such as, for example, the capacitance, the equivalent series resistance, the breakdown voltage or the residual current, but in particular the influencing of the equivalent series resistance, at low temperatures, in particular at temperatures down to below −40° C. The “life” of a capacitor is understood as meaning the influencing of the electrical characteristic values thereof, but in particular the influencing of the equivalent series resistance, after storage for several days, in particular after storage for 500 hours, at high temperatures, in particular at a temperature of 120° C.
The solid electrolyte capacitors which are known from the prior art, for example from EP-A-0 340 512, and are produced by in situ polymerization are characterized by a low equivalent series resistance and stable low temperature properties compared with the liquid electrolyte capacitors known from the prior art, but the life of these solid electrolyte capacitors is often still inadequate. There is therefore a demand for capacitors which show an improved life with at the same time a low equivalent series resistance and stable low temperature properties.